Dosimetric impact of density overrides on treatment planning for prostate MR-guided adaptive radiotherapy

Adaptive radiation therapy (ART) using magnetic resonance (MR)-guided linear accelerators (MR-LINACs) is an emerging approach that allows for daily treatment plan adaptation. However, bulk density overrides (BDOs), used during adaptive planning (e.g., on the Elekta Unity system), may introduce dosimetric uncertainties compared to reference computed tomography (CT) images by approximating electron densities (ED). This study aims to evaluate and quantify the dosimetric differences between BDO-based plans and corresponding reference CT-based plans within the Monaco treatment planning system (TPS) for prostate MR guided adaptive radiotherapy (MRgART). Using a retrospective analysis approach, 21 anonymized patients undergoing prostate volumetric modulated arc therapy (VMAT) were selected. CT numbers from the planning CTs were converted to electron densities using the CT-to-ED table in use at our institution. Dose from the original plan was calculated on the CT dataset in the Monaco TPS (used as reference). Copies of each plan were then made, in which a BDO was applied by assigning a homogeneous Hounsfield Unit (HU) value to each delineated structure. Dose was recalculated with the overridden densities (converted to ED via the same CT-to-ED table) and compared to the reference plan by baseline and QUANTEC (Quantitative Analysis of Normal Tissue Effects in the Clinic) dose-volume histogram (DVH) metrics, expressed as a percentage of the prescription dose. Regions with increased density heterogeneity, such as air in the rectum, exhibited the largest differences, with some baseline DVH metrics (D_50%) differing by up to 9%. No QUANTEC DVH metrics differed by more than 1%. The average of the mean dose difference to the prostate contour was 0.6%. No other contoured volumes of interest exceeded a 1.0% difference in mean dose. The findings of this study underscore the clinical viability of using BDOs for MRgART within the Elekta Unity / Monaco workflow. While there were localized dose discrepancies from the BDO, especially in the rectum, it did not affect the critical QUANTEC DVH metrics in any significant way. As this evaluation was performed on a single TPS platform, direct generalization to other systems (e.g., RayStation, Eclipse) should be made cautiously. Understanding the implications of BDO on dose distribution is crucial for optimizing treatment planning strategies and ensuring delivery of safe and efficacious radiation therapy in the context of MRgART.